Noninnervated embryonic chick hearts and isolated, mammalian, myocardial cells will be employed in studies to determine if phospholipids play a role in interactions with and responses to agents, e.g. isoproterenol, acetylcholine, angiotensin, histamine, whose effects are receptor-mediated. The capacities of selected commercial preparations of various phospholipases to impair or prevent responses (changes in rate and/or contractility) to these agents (agonists) will be determined. When such an effect is observed, the cardiac preparations will be exposed to the enzyme in the presence of a receptor agonist or blocking agent to determine if the effect of the enzyme can be prevented. This should indicate whether the enzyme is affecting the receptor in question or some other part of the myocardial cell. Those enzymes exhibiting an effect on receptor-mediated responses will be purified to determine if the effect of the commercial preparation can be ascribed to the enzyme in question of some other constituent of the relatively impure preparation. Purified enzymes will then be examined for their effects on the phospholipid current and composition of the cardiac preparation. This should provide some indication of whether or not the observed effect of the enzyme on the receptor is related to an alteration in its phospholipid composition.